1. Field of the Invention
The present invention relates to a frequency compensating circuit, and more particularly to a mixed type frequency compensating circuit.
2. Description of the Related Art
FIG. 1 is a circuit diagram illustrating a conventional DC-DC converter 100 including a voltage-amplifier-type frequency compensating circuit. The DC-DC converter 100 is a buck-type DC-DC converter that generates a DC output voltage lower than a DC input voltage.
The DC-DC converter 100 can include a DC voltage source VS, a switching transistor MN1, a diode D1, an inductor L1, a capacitor C1, a resistor RL, a frequency compensating circuit 110, a current detecting circuit 120, a comparator 130, an RS-type flip-flop 140, and a buffer 150.
The switching transistor MN1 can be driven in response to a gate driving signal VG, and provides the input voltage VIN to a node N11. The diode D1 can have a cathode coupled to the node N11 and an anode coupled to the ground voltage GND. The inductor L1 can be coupled between the node N11 and an output node N12. The capacitor C1 can be coupled to the output node N12 and the ground voltage GND. The resistor RL can be a load resistor, and coupled between the output node N12 and the ground voltage GND.
The frequency compensating circuit 110, the current detecting circuit 120, the comparator 130, the RS-type flip-flop 140, and a buffer 150 constitute a control circuit that can drive the switching transistor MN1 included in the DC-DC converter 100. The control circuit can generate a gate driving signal VG and provide the gate driving signal VG to a gate of the switching transistor MN1.
The frequency compensating circuit 110 can include an operational amplifier 111, feedback resistors RF1 and RF2, resistors RC1 and RC2, and a capacitor CC1. The frequency compensating circuit 110 can be a voltage-amplifier-type frequency compensating circuit, and amplify the feedback voltage signal VFB to generate a compensating voltage signal VC.
The current detecting circuit 120 can detect a current flowing through the switching transistor MN1 to generate a detecting voltage signal VSEN1. The comparator 130 can compare the compensating voltage signal VC with the detecting voltage signal VSEN1. An output signal of the comparator 130 can be applied to a reset terminal of the RS-type flip-flop 140, and a clock signal CLK to a set terminal of the RS-type flip-flop 140. The RS-type flip-flop 140 can perform pulse-width modulation on the output signal of the comparator 130. The buffer 150 can buffer an output signal of the RS-type flip-flop 140 to generate a gate driving signal VG. The gate driving signal VG can be applied to a gate of the switching transistor MN1.
The frequency compensating circuit 110 included in the DC-DC converter 100 can be a voltage-amplifier-type frequency compensating circuit. The capacitor CC1, that is inserted to compensate for a frequency characteristics, can have a small capacitance. The resistor RC1 can have a large resistance. For example, capacitor CC1 may have a capacitance of 10 pF and resistor RC1 may have a resistance of 15.9 MΩ. Capacitors with a capacitance of only 10 pF can be included in conventional semiconductor integrated circuits, because they require a relatively small chip area. However, it is difficult to include a resistor having a resistance of 15.9 MΩ in the semiconductor integrated circuit because such resistors occupy a larger chip area of the semiconductor integrated circuit.
The voltage-amplifier-type frequency compensating circuit 110 may be considered as a superposed circuit of an integral component sub-circuit 110a shown in FIG. 2A and a proportional component sub-circuit 110b shown in FIG. 2B.
In the integral component sub-circuit 110a, a capacitor CC1 can be coupled between an output terminal of the operational amplifier 11 and an inverted input terminal, but the resistor RC1 is absent. In the proportional component sub-circuit 110b, a resistor RC1 can be coupled between the output terminal of the operational amplifier 111 and the inverted input terminal, but the capacitor CC1 is absent.
FIG. 2C is a graph illustrating a voltage gain AV as a function of a frequency of the frequency compensating circuit 110 of FIG. 1. At low frequencies the voltage gain AV is approximately equal to a value Ao. For frequencies above a pole frequency fp the voltage gain AV decreases. Finally, above a zero frequency fz the voltage gain AV levels off at a value of RC1/RC2. The pole frequency fp may be expressed as fp=1/(2π·Ao·RC2·CC1), and the zero frequency fz may be represented as fz=1/(2π·RC1·CC1). Here, Ao denotes an open loop gain of the operational amplifier 111.
FIG. 3 illustrates a circuit diagram of a transconductance-amplifier-type frequency compensating circuit 210. The transconductance-amplifier-type frequency compensating circuit 210 may be used instead of the voltage-amplifier-type frequency compensating circuit 110 of FIG. 1.
The transconductance-amplifier-type frequency compensating circuit 210 may include a transconductance amplifier 211, feedback resistors RF1 and RF2, a resistor RC3, and a capacitor CC3. The feedback voltage VFB may be an output voltage VOUT of a DC-DC converter that is divided by the feedback resistors RF1 and RF2. The first transconductance amplifier 210 is a transconductance-amplifier-type frequency compensating circuit, and amplifies the feedback voltage signal VFB to generate a compensating voltage signal VC.
The resistor RC3, inserted to compensate for a frequency characteristics, can have a small resistance, but the capacitor CC3 can have a large capacitance. For example, RC3 may have a resistance of 191 kΩ and CC3 may have a capacitance of 830 pF. It is possible to include a resistor having a resistance of 191 kΩ in a semiconductor integrated circuit. However, it is more challenging to include a capacitor having a capacitance of 830 pF in the semiconductor integrated circuit because the capacitor occupies a large chip area of the semiconductor integrated circuit.
The transconductance-amplifier-type frequency compensating circuit 210 may be considered as a superposed circuit of an integral component sub-circuit 210a shown in FIG. 4A and a proportional component sub-circuit 210b shown in FIG. 4B.
In the integral component sub-circuit 210a, a capacitor CC3 can be coupled between an output terminal of the transconductance amplifier 211 and the ground voltage GND, but the resistor RC3 is absent. In the proportional component sub-circuit 210b, a resistor RC3 can be coupled between the output terminal of the transconductance amplifier 211 and the ground voltage, but the capacitor CC3 is absent.
FIG. 5 is a graph illustrating voltage gain AV as a function of a frequency of the frequency compensating circuit 210 shown in FIG. 3. At low frequencies the voltage gain AV is approximately gm·ro. Above a pole frequency fp the voltage gain AV decreases. Finally, above a higher zero frequency fz the voltage gain AV levels off with an approximate value of gm·RC3. The pole frequency fp may be expressed as fp=1/(2π·ro·CC3), and the zero frequency fz may be represented as fz=1/(2π·RC3·CC3). Here, gm denotes a transconductance of the transconductance amplifier 211, and ro denotes an output resistance of the transconductance amplifier 211.
In some conventional transconductance-amplifier-type frequency compensating circuits 210 the zero frequency fz is about 1 kHz when the transconductance gm of the transconductance amplifier 211 is about 200·10−6 A/V, RC3 is about 191 kΩ and CC3 is about 830 pF. The voltage gain AV may be expressed as gm·RC3, and has a value of about 31.6 dB (=20×log 38) at this zero frequency fz=1 kHz.
As described above, the compensating resistor RC1 included in the voltage-amplifier-type frequency compensating circuit 110 shown in FIG. 1 may have a resistance of about 15.9 MΩ. A lot of chip area is needed to implement a resistor having such a large resistance in the semiconductor integrated circuit. Further, the compensating capacitor CC3 included in the transconductance-amplifier-type frequency compensating circuit 210 shown in FIG. 3 may have a capacitance of about 830 pF. A lot of chip area is needed to implement a capacitor having such a large capacitance in the semiconductor integrated circuit.
Therefore, a frequency compensating circuit is needed, which may include a capacitor and a resistor, which can be integrated onto a smaller chip area in a semiconductor integrated circuit.